This proposal seeks to explore the scope of a newly discovered photoinduced cycloaddition of alkenes to excited triplet benzenes. The initial photoproducts are bicyclo [4.2.0]octa-2,4-dienes resulting from ortho 2 + 2 addition. These are converted thermally to cyclooctatrienes, which undergo photoisomerization to bicyclo ]4.2.0]octa-2,7,-dienes. Thus the overall chemistry can lead either to eight-membered rings or to two different bicyclic structures that would lead to a variety of alternative structures. The compounds that have been studied already are ortho- and para-butenoxy or pentenyl phenyl ketones; their intramolecular cyclization leads to bicyclic 8/5 ring fusions that occur in ophiobolins and related structures and also to tricyclic 4/6/5 ring fusions that occur in the rare sterpurene class of compounds. Both types of structures are of current synthetic interest because of their physiological properties. The long range goals of the research program are to define how well this photochemistry may complement the cycloaddition of alkenes to cycloalkenones and to singlet excited benzenes as Synthetic methodology; and to develop a much better understanding of the reactivity of excited triplet benzenes. Specific research aims for the next grant period are: 1) further define the regioselectivity for both intra- and intermolecular addition to substituted benzenes; 2) further define the diastereoselectivity of the reaction for substituents at various positions on the tether; 3) to explore enantioselectivity with various chiral ester or amide substituents; 4) measure triplet rate constants as a function of tether composition; 5) further explore effects of substituents on the double bond; 6) develop techniques to sensitize comparable reactions of benzoic acid derivatives and other aromatics; 7) characterize the thermal and photochemical reactions of the various polycyclic photoproducts 8) to test the reactions applicability to the synthesis of precapnelladiene and of sterpurene.